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Old 22nd July 2011, 04:30 AM   #1
Ryssen is offline Ryssen  Sweden
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Default Anyone used C3g to drive GM70?

Havent yet decided what tube to use to drive my GM70SE.I have loocket at the D3a
813 / GM70 amplifier with Bartolucci transformers
But I think the sensitivity is a little high 1vRMS for full audio signal.
My preamp/DAC gives about 3,5vp so I was thinking in those ranges.
The I thought of the C3g it has 40 in gain,but can it drive GM70 fully?
What would the input sensitivity be with C3g?
On GM70 I will use 950v/100mA -90v.
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Old 22nd July 2011, 07:14 AM   #2
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Shouldn´t be a problem with D3a as it only gives you a marging of just over 3dB. You will probably need that.
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Old 22nd July 2011, 12:25 PM   #3
Pyka is offline Pyka  China
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When connected in triode,D3a gain=80 .Use 1:1.5 interstage transformer.Total gain of D3a+interstage transformer >100.
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Old 22nd July 2011, 01:53 PM   #4
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Hey Ryssen,

Forgot to say sensitivity would be close to half ie. 2Vrms. So with 2,5V from your DAC it wouldn´t be much margin.

Also stick to 1:1 as this will probably give a sonically better result. You could then also go bifilar. Then check with Dave Slage for great irons!

Last edited by revintage; 22nd July 2011 at 01:58 PM.
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Old 22nd July 2011, 02:51 PM   #5
kevinkr is offline kevinkr  United States
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Working on my own GM70 design with triode connected D3A driver via Lundahl 1:1 IT. I have the impression that my design may be marginal in terms of both drive and bandwidth (on the low end).. My custom Edcor power transformers arrived a couple of weeks ago, and when this stupid, brutally hot summer weather goes away I will start on the PSU chassis (the amps themselves are finished) and report back on the results.

The recommendation for Slagle iron is a good one. (I know Dave personally, and am familiar with the quality of his products - good stuff. )

Worst case I will get some other ITs made with higher primary inductance, mine have ~30H and I think it is not going to be enough.. Running at just below 20mA...

I'm also thinking about experimenting with the D3A in pentode connection with RC coupling. The current set up is not capable of real A2 anyway so I would loose little IMHO..
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Old 22nd July 2011, 02:57 PM   #6
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What OPT do you use?

Slagles stuff is great, I have used quite a few I have custom-ordered(but they all are). About pentode D3a I remember Michael Koster have found 10k/20mA/150V/150V to be linear.

Last edited by revintage; 22nd July 2011 at 03:00 PM.
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Old 22nd July 2011, 03:01 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by revintage View Post
What OPT do you use?

Slagles stuff is great, I have used quite a few I have custom-ordered(but they all are).
I have a pair of custom 7K:8 150mA, 40W transformers Jack Elliano at Electra-Print made for me last year. The previous OPTs Jack made for me for a headphone amp worked so much better than I expected that I went back for more..
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Old 22nd July 2011, 03:02 PM   #8
kevinkr is offline kevinkr  United States
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Interesting comment on the operating point for pentode connected D3A, I was actually thinking of more or less those exact conditions with stiffly regulated screen of course.
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Old 22nd July 2011, 03:04 PM   #9
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Funny you mentioned Jack. Bought two pairs of custom transformers from him a while ago....
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Old 22nd July 2011, 04:50 PM   #10
Ryssen is offline Ryssen  Sweden
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Quote:
Forgot to say sensitivity would be close to half ie. 2Vrms. So with 2,5V from your DAC it wouldn´t be much margin.

Also stick to 1:1 as this will probably give a sonically better result.
I will go for the D3a then.And I will try capacistors first,maybe later when I can afford it, IT trafo,
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